1. Field of the Invention
The present invention relates to a bottle formed by biaxially stretch-blow-molding a thermoplastic resin and to a method of producing the same. More particularly, the invention relates to a bottle that is particularly oriented in the bottom portion thereof and has excellent resistance against environmental stress (ESC), shock resistance, strength and appearance in the bottom portion thereof, and to a method of producing the same.
2. Description of the Prior Art
Stretch-blow-molded plastic containers composed of polyesters or polypropylenes and, particularly, polyester containers have nowadays been widely used for containing liquid products such as liquid detergents, shampoos, cosmetics, sauces such as soy sauces, etc., as well as for containing aerated beverages such as beer, Coke, cider, etc., for the other containing beverages such as juices, mineral water, etc. and have also been used as cups for containing desserts, miso and other foods, owing to their excellent transparency and suitable degree of gas barrier property.
The methods of producing polyester bottles can be roughly divided into the hot parison method and the cold parison method. According to the former hot parison method, the preform formed by injection-molding the polyester is stretch-blow-molded in a hot state without being completely cooled. According to the latter cold parison method, on the other hand, a polyester is injection-molded to, first, form an overcooled preform with bottom of amorphous polyester having a size considerably smaller than a final container, the preform is pre-heated at a stretching temperature, and is stretched in the axial direction in a blowing metal mold and is stretched by blowing in the circumferential direction.
The preform with bottom has a mouth-and-neck portion corresponding to the mouth-and-neck portion of a container and a cylindrical portion with bottom that is stretch-blow-molded, and generally has the shape of a test tube as a whole when the container is a vertically elongated one. In the mouth-and-neck portion, for example, are formed an open end for being sealed and a means for engagement with a closure. In the bottom is further necessarily formed a gate portion that protrudes outward from the center of the bottom for conducting the injection-molding.
In the polyester bottles that have heretofore been stretch-blow-molded, the orientation and crystallinity are most disturbed in the bottom portion of the bottle and, particularly, at the central portion thereof, causing problems such as deteriorated appearance and drop in the bottle properties.
For instance, even though the barrel portion of the PET bottle is completely transparent, the gate portion and the peripheries thereof of the preform are often whitened as generally called gate whitening. It has been said that this phenomenon occurs since the PET resin that is distorted due to orientation by fluidization has a property of being very easily crystallized.
In order to prevent the gate whitening, the molding must be conducted under the conditions where the residual strain due to fluidization does not increase. For example, it has been known that the gate whitening takes place little as the gate diameter increases or as the injection pressure decreases.
As the gate diameter increases, however, the diameter of the remaining portion of the gate increases, too, in the bottom portion of the bottle deteriorating the appearance. As the injection pressure decreases, further, the preform is poorly molded and the injection retention time is lengthened, resulting in a decrease in the productivity.
The present inventors have discovered that the strain due to orientation by fluidization taking place at the time of injection-molding the preform still adversely affects the bottom portion of the bottle being uniformly and homogeneously stretched, adversely affects the resistance against cracking due to environmental stress and the strength of the bottle even when the strain due to orientation by fluidization is on such a low level that does not cause the gate whitening.
The object of the present invention is to provide a bottle without substantially strain due to orientation by fluidization in the bottom portion, the bottom portion being uniformly and homogeneously stretched, having improved shock resistance and buckling strength, having excellent resistance against cracking due to environment at the center of the bottom portion, without developing crazing or whitening during the preservation, and exhibiting excellent appearance.
According to the present invention, there is provided a bottle formed by biaxially stretch-blow-molding and having a mouth portion, a shoulder portion, a barrel portion and a bottom portion, said bottom portion without being substantially affected by the residual strain due to orientation by fluidization.
In the present invention, it is desired that:
(1) the bottle is formed by biaxially stretching-blow-molding a preform that is formed by compression-molding a thermoplastic resin;
(2) the preform has a difference (tmaxxe2x80x94tmin) between a maximum thickness and a minimum thickness in the barrel portion thereof in the circumferential direction, which is not larger than 0.07 mm;
(3) when the bottle is composed of a thermoplastic polyester, the central portion in the bottom portion of the bottle has a diffuse scattering peak in 2 xcex8 of from 19.45 to 20.50xc2x0 as measured by using X-rays (Cu-xcex1) falling in the direction of thickness of the container wall, a peak position (A) at a portion on the outer surface side of the central portion in the bottom portion is an angle lower than that of a peak position (B) at a portion on the inner surface side of the central portion in the bottom portion, the difference (Bxe2x88x92A) thereof is not smaller than 0.15 degrees, a half-value width (C) of the diffuse scattering peak of X-rays in the portion on the outer surface side at the central portion in the bottom portion is larger than a half-value width (D) of the diffuse scattering peak of X-rays in the portion on the inner surface side at the central portion in the bottom portion, and the difference (Cxe2x88x92D) thereof is not smaller than 0.10 degrees; and
(4) the mouth portion has a support ring, and an annular groove is formed in the inner peripheral edge in the lower surface of the support ring.
According to the present invention, there is further provided a method of producing a bottle by biaxially stretch-blow-molding a pre-molded article for blow molding that is obtained through the steps of:
extruding a molten thermoplastic resin and cutting it into a mass of melt of nearly a predetermined amount; arranging a male metal mold and a female metal mold so as to move relative to each other and supplying the mass of melt into the metal mold;
compression-molding the mass of melt into a pre-molded article having a barrel portion with bottom and an annular groove formed in the inner peripheral edge in the lower surface of the support ring; and
cooling and solidifying the compression-molded article and discharging the molded article out of the metal mold.
According to the method of the present invention, it is desired that:
(5) the mass of melt is compression-molded into a pre-molded article while discharging the air remaining in the metal mold;
(6) fine gaps or holes are formed in a portion of the metal mold forming the bottom portion of the pre-molded article; and
(7) the male metal mold is constituted by a core metal mold and a follower metal mold surrounding the core metal mold and is allowed to be opened and closed coaxially therewith, the barrel portion with bottom is molded by the core metal mold and the female metal mold, and the mouth portion is molded by the core metal mold and the follower metal mold.